Interpretive Summary: The molds that contaminate some crops with the toxic and cancer-causing chemical called aflatoxin are usually found in the soil. The molds belonging to the species Aspergillus flavus cannot mate sexually. They possess a type of immune system that prevents sexual exchange of genetic material. In spite of this previously, other researchers reported that these fungi are able to undergo what they classified as “cryptic sex” and, because of this, can exchange genetic material. We show in our study that molds of the same category (vegetative compatibility group [VCG]) do not genetically cross with molds of a different VCG. We show this by isolating molds of the same VCG from soils of Arizona, Texas, Louisiana and Mississippi, and show that in spite of the large geographical separation, molds of the same type (VCG) are indeed the same, and moreover, are different from molds of another VGG. This study proves that genetic exchange by so-called cryptic sex, if it is occurring at all, is occurring at a very low frequency. This is important because it proves that if soils are inoculated with an aflatoxin-non-producing fungus, a Frankenstein strain that could be even more dangerous to the environment than any of the natural strains, will be produced.

Technical Abstract:
Genetic exchange among populations of asexual filamentous fungi is presumed to be limited to isolates in the same vegetative compatibility group (VCG). To test this hypothesis, we compared the distribution of single nucleotide polymorphisms (SNP's) in Aspergills flavus isolates from six different VCG's obtained from locations in the southern United States separated by as much as 1200 km. Based on DNA sequence analysis of three non-contiguous gene regions, all isolates from the same VCG possessed the same defining SNP's, a result indicating an absence of recombination among isolates from dfferent VCG's.